test: enhance return of the doctest script

tests.py

@@ -1,6 +1,18 @@
+#!/usr/bin/env python3
+
 import doctest
 
 from kosmorrolib import *
 
-for module in [events, ephemerides]:
-    doctest.testmod(module, verbose=True)
+
+if __name__ == '__main__':
+    failures = 0
+    tests = 0
+
+    for module in [events, ephemerides]:
+        (f, t) = doctest.testmod(module)
+        failures += f
+        tests += t
+
+    if failures == 0:
+        print("✔ All %d tests successfully passed." % tests)